The improvement in the impact strengths of polyamides is generally obtained by incorporation, in the form of a dispersed phase, of an impact modifier which is elastomeric in nature and has reactive functional groups (acrylic acid, maleic anhydride, etc.) capable of reacting with the functional groups, such as the NH2 chain ends, of the polyamide matrix. This reactivity makes it possible to ensure a fine homogeneous dispersion of the elastomer by controlling the size of its particles or nodules, a good adhesion at the nodule/matrix interface, and to obtain an optimum increase in the impact strength. However, the drawback of this technique is the thickening of the blend, due to the PA-impact modifier coupling, which leads to a significant fall in the melt flow. This large change in the viscosity is detrimental to the processing of such compositions, especially for thin or large-sized injection-moulded parts and also an extrusion.
Descriptions have already been given in the prior art of numerous polyamide-based impact-resistant compositions.
EP 96 264 describes polyamides with a relative viscosity of between 2.5 and 5 that are reinforced by ethylene/C2 to C8 alkyl (meth)acrylate/unsaturated acid or anhydride copolymers and comprising 20 to 40% by weight of acrylate.
EP 2 761 describes polyamides reinforced by polyethylenes or copolymers of ethylene grafted with (meth)acrylic acid, its derivatives or maleic anhydride and optionally polyethylene.
EP 52 796 describes polyamides reinforced by (i) an α-olefin/unsaturated carboxylic acid copolymer, (ii) an α-olefin/alkyl ester of an unsaturated carboxylic acid copolymer and (iii) a metal compound.
FR 2 292 016 describes polyamides reinforced by a blend of copolymers of α-olefins and of esters of unsaturated carboxylic acids or their derivatives.
U.S. Pat. No. 5,070,145 describes polyamides reinforced by a blend (i) of a polyethylene or of an ethylene/alkyl (meth)acrylate copolymer and (ii) of an ethylene/alkyl (meth)acrylate/maleic anhydride copolymer.
U.S. Pat. No. 4,174,358 describes reinforced polyamides that are in the form of a polyamide matrix dispersed in which are nodules having a size between 0.01 and 1 μm, having a modulus between 0.007 and 138 MPa, the ratio of the modulus of the PA matrix to that of the impact modifier having to be at least equal to 10. A great many reinforcing agents are described, some having epoxide functional groups (but no examples are given), most are polymers having neutralized anhydride or acid functional groups or are mixtures based on EPDM. All these reinforcing agents are presented as equivalent.
This entire prior art gives examples only of reinforcing agents having acid or acid anhydride functional groups or EPDM, and no mention is made of the viscosity of the reinforced polyamides. It will be seen in the comparative examples of the present invention that the behaviour of the modifiers of the prior art has no connection with that of the modifiers of the present invention.
EP 802 226 by the Applicant describes injection-moulded parts based on polyamide resin (A) and on an amount of a copolymer (B) of ethylene and of an unsaturated epoxide such that their melt flow index is greater than the ratio of the melt flow index of the polyamide (A) divided by 0.175 times the weight percentage of the copolymer (B).
EP 821 037 by the Applicant describes compositions comprising at least one polyamide (A) and at least one copolymer (B) of ethylene and of at least one unsaturated carboxylic acid anhydride (or acid), the amount of which is between 0.1 and 0.5% by weight, such that the impact/flow index (IF) of the compositions comprising 90 parts of (A) per 10 parts of (B) is greater than 1.5, these compositions containing essentially no glass fibres. These documents do not describe the use of impact modifiers as a mixture with low density polyethylenes.
EP 564 338 describes polyamides reinforced (i) by ethylene/alkyl (meth)acrylate/glycidyl methacrylate copolymers and (ii) optionally by polyethylenes, ethylene/alkyl (meth)acrylate copolymers or ethylene/alkyl (meth)acrylate/unsaturated dicarboxylic acid anhydride copolymers. This prior art is only interested in the impact strength and not in the viscosity. It is clear (from the table of examples) that it is advantageous to mix a reinforcing agent having epoxide functional groups with a reinforcing agent having anhydride functional groups in order to cause crosslinking of the dispersed phase, which gives significant impact strength but to the detriment of the viscosity.
WO 94/25526 describes a composition comprising a homogeneous blend of at least one thermoplastic polymer, of polyester or polyamide type, and a copolymer of substantially linear ethylene, obtained by metallocene catalysis, having a density greater than 0.850 g/cm3, grafted with at least 0.01% by weight of a maleic anhydride type unsaturated compound. The optimum degree of grafting is between 0.05 and 10% by weight.
The linear polyethylene contains from 95 to 50% by weight of ethylene and from 5 to 50% by weight of C3 to C20 comonomer, in particular of 1-octene, with densities ranging from 0.860 to 0.935 g/cm3. Blends of PA-6,6 and of the grafted ethylene copolymer, at contents between 20 and 35% by weight of the blend, are exemplified and make it possible to improve its impact strength. No supplementary compatibilizer is used.
U.S. Pat. No. 6,303,688 describes a composition containing:    a) from 5 to 50% of:            i) 10 to 70% of a copolymer of ethylene and of C3 to C18 α-olefins, of the metallocene type, grafted between 0.1 and 10% by an unsaturated carboxylic acid or its anhydride;        ii) 90 to 30% of an elastomeric copolymer of ethylene and of C3 to C8 α-olefins (in particular an EPR, EPDM, or SBS), and optionally of an unconjugated diene;            b) from 95 to 50% of a thermoplastic polymer such as: PA, polyesters, polyacetals, polycarbonate; this composition has improved impact resistance properties, relative to the preceding document, by virtue of the use, as an impact modifier, of a blend of a grafted ethylene copolymer and of an ungrafted elastomer. No supplementary compatibilizer is used.
U.S. Pat. No. 3,963,799 describes a blend comprising from 60 to 90 parts of polyamide and from 40 to 10 parts of polyethylene (including high density polyethylene), introduced into which are 1.8 to 8 parts of a PA-block graft copolymer made from a backbone composed of a copolymer of ethylene and of a comonomer of the maleic anhydride type, which acts as a compatibilizer making it possible to disperse the polyethylene as nodules having a diameter between 0.2 and 1 μm. No indication on the viscosities of the blends obtained is given.
Document WO 02/28959 by the Applicant describes a polyamide-block graft copolymer in a polyolefin backbone which is chosen from ethylene/maleic anhydride and ethylene/alkyl (meth)acrylate/maleic anhydride copolymers, forming a nanostructured cocontinuous blend, which gives this copolymer excellent thermomechanical properties that are retained when redispersing this graft copolymer in flexible polyolefins such as flexible ethylene polymers.
Blends of this kind find applications as adhesives, films, tarpaulins, calendered products, electrical cables or powders for moulding processes (“slush-moulding”).
The Applicant surprisingly has discovered, that by combining, as a blend, this type of graft copolymer with at least one miscible non-reactive olefinic polymer, in large proportions, an impact-modifying compound was obtained, which when introduced into a polyamide-based composition, made it possible to obtain both an improved impact strength and a good melt flow allowing, in particular these compositions to be injection-moulded for producing various types of parts, or to be extruded.
This is especially the case for thin or larger-sized parts or alternatively parts with a complicated shape, these parts having to be produced at industrial output rates.
Generally, the impact properties of a polyamide are improved by the addition of a dispersed phase of an impact modifier which contains reactive functional groups (acrylic acid, maleic anhydride, etc.). The impact modifier used in our invention (blend of a graft copolymer with a non-reactive olefinic polymer) does not contain reactive functional groups. However, surprisingly, this modifier makes it possible to obtain a polyamide-based composition having excellent impact properties, in particular at low temperature. These low temperature properties are particularly sought after, for example in the automotive field where external parts made of polyamide (wing mirrors, etc.) must retain their impact properties whatever the outside conditions.
Furthermore, the use of an impact modifier generally leads to significant thickening of the composition. However, in the case of the impact modifier used in our invention, the composition remains more fluid than for other impact modifiers.
Without wanting to be tied to one theory in particular, it appears that the compatibilization takes place via miscibility of the PA grafts of the graft copolymer with the PA chains of the matrix, the anchoring of the elastomeric nodules being thermodynamic.
The advantage of the compositions of the invention is the compromise between the impact strength and the viscosity: these compositions combine excellent impact behaviour and a much higher melt flow of the polyamide-based formulations in comparison to the impact modifiers generally used.
Another optional advantage is that it is possible to add a single product to the polyamide resin and not a mixture of various products.
Another advantage is that it is sufficient to add the PA grafted ethylene copolymer into the extruder which feeds the injection-moulding device. This addition can be made via a side extruder or a granule metering device on the polyamide extruder. It is therefore not necessary to prepare the blend of the polyamide resin and of the impact reinforcing agent in advance.